This application relates to the design of a gas turbine engine rotor which can be operated to produce noise that is less sensitive to human hearing.
Gas turbine engines are known, and typically include a fan delivering air into a compressor. The air is compressed and delivered downstream into a combustor section where it was mixed with fuel and ignited. Products of this combustion pass downstream over turbine rotors, driving the turbine rotors to rotate.
In a two spool engine, there is a high pressure turbine rotor, and a low pressure turbine rotor. Each of the turbine rotors include a number of rows of turbine blades which rotate with the rotor. Interspersed between the rows of turbine blades are vanes.
The high pressure turbine rotor has typically driven a high pressure compressor rotor, and the low pressure turbine rotor has typically driven a low pressure compressor rotor. Each of the compressor rotors also include a number of compressor blades which rotate with the rotors. There are also vanes interspersed between the rows of compressor blades.
The low pressure turbine or compressor can be a significant noise source, as noise is produced by fluid dynamic interaction between the blade rows and the vane rows. These interactions produce tones at a blade passage frequency of each of the low pressure turbine rotors, the low pressure compressor rotors, and their harmonics.
The noise can often be in a frequency range that is very sensitive to humans. To mitigate this problem, in the past, a vane-to-blade ratio has been controlled to be above a certain number. As an example, a vane-to-blade ratio may be selected to be 1.5 or greater, to prevent a fundamental blade passage tone from propagating to the far field. This is known as “cut-off.”
However, acoustically cut-off designs may come at the expense of increased weight and reduced aerodynamic efficiency. Stated another way, by limiting the designer to a particular vane to blade ratio, the designer may be restricted from selecting such a ratio based upon other characteristics of the intended engine.
Historically, the low pressure turbine has driven both a low pressure compressor section and a fan section. More recently, a gear reduction has been provided such that the fan and low pressure compressor can be driven at distinct speeds.
In another type of gas turbine engine, there are three turbines, with a higher pressure turbine driving the high pressure compressor. An intermediate turbine drives the low pressure compressor. A low pressure turbine drives the fan. Such engines will also have the noise problems as discussed above.
It is known to provide a particular relationship between the number of blades in a low pressure compressor, or in the low pressure turbine, and the speed of an associated rotor. No such relationship has ever been extended to the intermediate turbine, which would be spaced further into the engine from either the low pressure compressor or low pressure turbine.